During the past several years, the use of three piece cans made from tin-free steel and featuring a side seam bonded with an adhesive has increased dramatically in the container industry, particularly in containers for beverages (i.e., beer, carbonated and non-carbonated soft drinks, fruit juices and the like). Thus, literally billions of these containers have been used by brewers and other beverage packagers. One of the reasons why such containers have proven so popular is that their adhesive bonded side seam construction has essentially eliminated the necessity for the old side seam soldering process. This feature provides the mechanism for high speed bonding of the side seam and, consequently, permits much greater speed and efficiency in container line production.
However, while such containers have proven to be highly advantageous, their construction and method of preparation impose greater requirements on coating materials and particularly the base coats and size coats utilized in such containers. Thus, these three piece containers are generally prepared by a process which involves the steps of first passing a sheet of metal container body stock (i.e., tin-free steel) usually through a roll coater to apply the base coat composition; curing the base coat composition by passing the coated sheet through an oven and baking at elevated temperatures (i.e., 400.degree. F. or above); turning the basecoated sheet over to apply the size coat; applying the size coat, usually by flowcoating; curing the size coat by passing the sheet through an oven and baking at elevated temperatures; applying decorative printing; passing the sheet stock to a splitter which cuts the sheet into can body blanks the size of the desired can cylinder; heating the body blank; applying a thermoplastic adhesive (e.g. thermoplastic polyamide) to one edge of the body blank; passing the body blank edge coated with adhesive to the body maker; reheating to elevated temperature (e.g., 475.degree. F.) the body blank having previously applied adhesive; wrapping the heated body blank around a mandrel and then bumping the seam together with chilled tools operating at about -25.degree. F. to bond the lap seam. Following this process, a protective coating is applied to the exposed cut edges of the can cylinder, the can cylinder is flanged, the interior of the can is sprayed with a topcoat and after curing the topcoat, one of the can ends is applied and seamed. At the conclusion of this process, the container is then ready to be packaged with beverage.
As is evident from the above brief description of the process, the base coats and size coats employed in the container are subject to strenuous processing conditions, including mechanical stress, varying temperatures and the like. In order to perform successfully in such an environment, base coat and size coat compositions must meet a number of difficult requirements. For example, such coatings must have excellent adhesion to metal (i.e., tin-free steel); excellent fabricating properties, i.e., a combination of flexibility, extensibility and adhesion so as to permit forming operations to be carried out on the coated metal without cracking or otherwise impairing the continuity of the film; excellent intercoat adhesion (e.g., adhesion to topcoat material); excellent adhesion to the bonding adhesive employed on the lap seam; and resistance to the high temperatures employed in container production or during pastuerization of the beverage (e.g., beer). In addition to the foregoing properties, while the size coat (exterior) and base coat (interior) may not be in direct contact with the packaged beverage, the possibility exists that due to incomplete topcoat coverage or accidental exposure in the seam area these coatings should also be formulated so that they do not adversely affect the taste characteristics of or cause turbidity in the packaged beverage.
As will be recognized from the above, the production of base coat and size coat materials meeting all of these requirements is a difficult undertaking. Furthermore, the problem of producing successful coating materials of that type has been even further complicated by the increasing national emphasis on ecological and environmental concerns. Due to these concerns, the coating and container industries today are vitally interested in replacing volatile organic solvent-based coating materials with aqueous coatings which eliminate or substantially minimize air pollution caused by solvent emissions during cure of the coating. To produce aqueous base coat and size coat materials for use in the three piece containers described above has proven to be a difficult task.
In accordance with this invention, a coating composition meeting all of the requirements for base coats and size coats in three piece tin-free steel beverage containers having adhesive bonded side seams has now been found. Thus, the base coat and size coat compositions of this invention have excellent adhesion to metal, excellent intercoat adhesion, excellent adhesion to seam bonding adhesive; good high temperature resistance; and do not adversely affect packaged beverages.